1. Field of the Invention
The invention relates to a method for processing an image wherein image information is processed depending on a dominant direction of image structure in the image.
2. Description of the Related Art
Such a method for processing an image is known from the German Offenlegunsgschrift DE 44 31 349.
Such a method is in particular suitable to be applied to a medical diagnostic image which is e.g. obtained by magnetic resonance imaging (MRI), x-ray imaging or x-ray computed tomography (CT). Such an image contains a noise component which obscures small details with only little contrast. However, such small details in the image can be of particular medical relevance, viz. small deviations such as tumours in an early stage of the pathology should be made visible in a rendition of a medical diagnostic image. In x-ray imaging, the quantum shot noise of the x-rays is an important cause for noise in the image. In MRI imaging a dominant source of noise is formed by electronic noise in the system which receives radio-frequent magnetic resonance signals which represent relaxation of excited nuclear spins in the body of the patient.
I. The method disclosed in the cited reference aims at improving the quality of MRI images. According to the known method for processing an image, four sub-images are formed by low-pass directional filtering the input image. Each of the sub-images is obtained from the input image by smoothing the pixel-values along four respective predetermined directions in the image are formed. Subsequently, difference images which represent the difference between the input-image and the respective sub-images. Pixel-values of respective positions in the processed image are obtained in the form of weighted averages of pixel-values of the corresponding positions of the respective sub-images are formed. The weights are derived as decreasing functions of pixel-values of said difference images. Structures in the image that run along the predetermined directions are substantially retained, while a noise component of the processed image is reduced as compared to the input image.
A drawback of the known method for processing an image is that the preservation of image structures relies on the limited number of predetermined directions in that image structures which run along a direction between two of the predetermined direction are less well preserved than image structures along a predetermined direction. Thus, when the known method of processing an image is applied to a medical diagnostic image, small but elongated details may become badly visible in a rendition of the processed image. Moreover, especially in three or more dimensional images the known method requires many predetermined directions and thus a huge computational effort to achieve an adequate improvement of the image quality.